1. Field of the Invention
The invention relates generally to the field of well logging. More particularly, the invention relates to improved techniques in which instruments equipped with antenna systems having transverse or tilted magnetic dipole-moment representations are used for electromagnetic measurements of subsurface formations and for defining the reservoir bedding structure and formation dip, as well as placing wells with respect to geological boundaries in a reservoir.
2. Background of the Related Art
Information that characterizes dips within a subsurface formation of interest is important for understanding the deposition environment of the sedimentary rocks, and for the development and execution of a well drilling plan for oil and gas exploration. The dip and strike of a formation bed can be extracted from seismic maps and from borehole images. Seismic maps provide large-scale structural information, and borehole images provide information related to the local formation environment penetrated by the borehole. Both information types are useful information for hydrocarbon prospecting. Dip information extracted from borehole images, however, is usually of higher accuracy than that extracted from seismic maps.
Various well logging techniques are known in the field of hydrocarbon exploration and production for evaluating the subsurface formation penetrated by a borehole. These techniques typically use instruments or tools equipped with sources adapted to emit energy into the formation. In this description, “instrument” and “tool” will be used interchangeably to indicate, for example, an electromagnetic instrument (or tool), a wire-line tool (or instrument), or a logging-while-drilling tool (or instrument). The emitted energy interacts with the surrounding formation to produce signals that are then detected and measured by one or more sensors. By processing the detected signal data, an image or profile of the formation properties is obtained.
Commercial tools presently offered for producing electrical borehole images include the GeoVision Resistivity (GVR) tool and the Azimuthal Density Neutron (ADN) tool (both “while drilling” tools) and the Formation Microresistivity Imager (FMI) tool (a wireline tool), all owned and offered through logging services by Schlumberger, the assignee of the present invention. Dips are extracted from borehole images by identifying bed boundary interfaces on the image or by determining correlations between images measured at different sensors. The accuracy of the dip estimate from images is affected by many factors including the quality of the images, the vertical resolution of the tool, the skills of the geologist, and—in deviated wells—the accuracy of the borehole survey.
Among the above-mentioned imaging tools, the FMI tool provides the highest quality wellbore images due to its employment of measurement electrodes having small sizes (e.g., 0.2-inches). The accuracy of the apparent dip from the FMI tool's images is typically around 0.5° for typical high dip angles (or dip heights). For lower apparent dip, the accuracy degrades to several degrees. Furthermore, the FMI tool and other electrode tools work only in conductive mud.
The GVR tool provides real-time dip services, but only for apparent dips larger than 53°. Analysis of the obtained real-time image at the surface can remove this restriction, but since the image is acquired using one-inch electrode buttons, the quality of the image does not permit accurate determination of dip when relative dip is low. The fast rate of penetration can also affect the image quality and thus dip accuracy from the image. Like the FMI tool, the GVR tool works only in conductive mud.
For oil-based and synthetic muds, the Oil Base MicroImager (OBMI) tool, also by Schlumberger, may be used to provide image services. The quality of the image is poorer than that of the FMI tool, and the error on determined dips will be larger than that of the FMI tool. Currently, no electric image tools provide dip services in both conductive and insulating mud.
Electromagnetic (EM) induction and propagation logging are well-known techniques. The logging instruments are disposed within a borehole on a wireline or via a drill string “while drilling” to measure the electrical conductivity (or its inverse, resistivity) of earth formations surrounding the borehole. In the present description, any reference to conductivity is intended to encompass its inverse, resistivity, or vice versa. A typical electromagnetic resistivity tool comprises a transmitter antenna and one or more (typically a pair) receiver antennas disposed at a distance from the transmitter antenna along the axis of the tool (see FIG. 1).
Induction tools measure the resistivity (or conductivity) of the formation by measuring the voltage induced in the receiver antenna(s) as a result of magnetic flux induced by AC currents flowing through the emitting (or transmitter) antenna. So-called propagation tools operate in a similar fashion, but typically at higher frequencies than do induction tools for comparable antenna spacings (about 106 Hz for propagation tools as compared with about 104 Hz for the induction tools). A typical propagation tool may operate at a frequency range of 1 kHz –2 MHz.
Conventional transmitters and receivers are antennas formed from coils comprised of one or more turns of insulated conductor wire wound around a support. These antennas are typically operable as sources and/or receivers. Those skilled in the art will appreciate that the same antenna may be used as a transmitter at one time and as a receiver at another. It will also be appreciated that the transmitter-receiver configurations disclosed herein are interchangeable due to the principle of reciprocity, i.e., the “transmitter” may be used as a “receiver,” and vice-versa.
The antennas operate on the principle that a coil carrying an AC current (e.g., a transmitter coil) generates a magnetic field. The electromagnetic energy from the transmitter antenna of a logging tool disposed in a borehole is transmitted into the surrounding regions of the formation, and this transmission induces an eddy current flowing in the formation around the transmitter (see FIG. 2A). The eddy current induced in the formation, which is a function of the formation's resistivity, generates a magnetic field that in turn induces an electrical voltage in the receiver antennas. If a pair of spaced-apart receivers is used, the induced voltages in the two receiver antennas would have different phases and amplitudes due to geometric spreading and absorption by the surrounding formation. The phase difference (phase shift, Φ) and amplitude ratio (attenuation, A) from the two receivers can be used to derive the resistivity of the formation. The detected phase shift (Φ) and attenuation (A) depend on not only the spacing between the two receivers and the distances between the transmitter and the receivers, but also the frequency of EM waves generated by the transmitter.
In conventional induction and propagation logging instruments, the transmitter and receiver antennas are mounted with their axes along the longitudinal axis of the instrument. Thus, these tools are implemented with antennas having longitudinal magnetic dipole-moments (LMD). FIG. 2A presents a simplified representation of electromagnetic (EM) energy flowing from such a logging instrument disposed in a borehole portion or segment that penetrates a subsurface formation in a direction perpendicular to a formation bed of interest. This is not, however, an accurate depiction of all the numerous segments that make up a borehole—particularly when the borehole has been directionally-drilled as described below. Thus, segments of a borehole often penetrate formation layers at an angle other than 90 degrees, as shown in FIG. 2B. When this happens, the formation plane is said to have a relative dip. A relative dip angle, θ, is defined as the angle between the borehole axis (tool axis) BA and the normal N to the plane P of a formation bed of interest.
It is well known that the response of a logging tool will be affected by the formation bedding structures surrounding the segment of the borehole in which the tool is disposed. For electromagnetic logging tools, this is known as the shoulder bed effect. Accordingly, the responses of conventional induction and propagation tools having LMD antennas are affected by the formation bedding and its dips. However, such tools are inherently non-directional and, therefore, are incapable of providing azimuthal information about the bedding structure. Thus, commercially available wireline induction and LWD propagation resistivity tools are presently unable to accurately determine dip.
An emerging technique in the field of well logging is the use of instruments including antennas having tilted or transverse coils, i.e., where the coil's axis is not parallel to the longitudinal axis of the tool or borehole. These instruments are thus implemented with a transverse or tilted magnetic dipole-moment (TMD) antenna.
Those skilled in the art will appreciate that various ways are available to tilt or skew an antenna. Logging instruments equipped with TMD antennas are described, e.g., in: U.S. Pat. Nos. 6,163,155; 6,147,496; 5,115,198; 4,319,191; 5,508,616; 5,757,191; 5,781,436; 6,044,325; and 6,147,496. The response of such tools will depend on the azimuthal orientation of the tool in a dipping formation. Therefore, useful information about earth structure, in particular the dip and strike, can be obtained from a proper analysis of azimuthal or directional measurements.
U.S. Patent Application Publication No. 2003/0055565 to Omeragic, presently assigned to Schlumberger, derives closed-form expressions for the calculation of anisotropic formation parameters from tri-axial induction measurements. U.S. Pat. No. 6,163,155 to Bittar, assigned to Dresser, discloses a method and apparatus for simultaneously determining the horizontal resistivity, vertical resistivity, and relative dip angle for anisotropic earth formations by software rotation of orthogonal coils to achieve decoupling between the horizontal and vertical resistivity. U.S. Pat. No. 6,556,016 to Gao et al, assigned to Halliburton, discloses an induction method for determining approximate dip angle of anisotropic earth formation utilizing tri-axial measurements. These applications are limited to formations with anisotropy.